What Are Viruses? The Living/Non-Living Debate
Viruses occupy a uniquely ambiguous position in biology , they possess genetic material and can reproduce, yet they lack virtually every other characteristic we associate with life. Understanding why this is the case is central to A2.3.
Virus: A non-cellular infectious agent consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid), sometimes surrounded by a lipid envelope. Viruses can only replicate inside a living host cell.
The core reason viruses blur the line between living and non-living is their complete absence of metabolism. They carry no ribosomes, no enzymes for energy production, and no cell membrane with transport proteins. Outside a host cell, a virus is essentially an inert chemical package.
Think of a virus like a USB drive containing a programme. The drive itself does nothing , no power, no processing. But plug it into a computer (the host cell), and it hijacks the system to run its code and copy itself. The USB drive is not "alive," but its instructions certainly have an effect.
Because viruses lack metabolic machinery, they cannot:
- Generate their own ATP or energy
- Synthesise proteins independently
- Replicate without commandeering a host cell's ribosomes and enzymes
The lack of metabolism is why viruses are often described as obligate intracellular parasites , they are entirely dependent on the biochemical machinery of a host cell to reproduce.
Viral Structure: Genome, Capsid, and Envelope
All viruses share a minimal but highly organised structure. Understanding this structure helps explain both their function and why treating viral infections is so challenging.
Core components of every virus:
- Genome , either DNA or RNA (never both), which may be single-stranded or double-stranded, linear or circular.
- Capsid , a protein coat made of repeating subunits called capsomeres that encloses and protects the genome.
- Envelope (some viruses only) , a lipid bilayer derived from the host cell membrane, studded with viral glycoproteins. Not all viruses have this layer.
Enveloped viruses (e.g. HIV, influenza) are generally more sensitive to desiccation and disinfectants than non-enveloped viruses, because detergents and alcohol can dissolve the lipid envelope. This is why hand sanitiser is effective against many enveloped viruses.
Key point for HL: The type of genome a virus carries has major implications for how it replicates. RNA viruses like HIV require the enzyme reverse transcriptase to convert their RNA genome into DNA before it can be integrated into the host's genome , a process that introduces many replication errors and drives rapid mutation.
